1. Field of the Invention
The present invention relates to a driving apparatus for ultrasonic motors, which is designed to drive a mover by means of the progressive oscillating waves produced in an elastic body by a piezoelectric body.
2. Related Background Art
An ultrasonic motor utilizing progressive oscillating waves is, as disclosed in Japanese Patent Application laid-Open No. 59-111609, a motor in which AC driving voltage is applied to a piezoelectric body to produce flexure oscillation in said piezoelectric body. This, in turn, produces progressive oscillating waves in an elastic body to which said piezoelectric body is attached, and a mover is pressed to come in contact with the elastic body, thus driving the motor by friction.
FIG. 6 is a cross-sectional view showing the outline of the configuration of a rotary ultrasonic motor, and FIG. 6 is a plan view of the ultrasonic motor observed from the piezoelectric body side.
Referring to the ultrasonic motor shown in FIGS. 6 and 7, a piezoelectric body 1004 is attached to one side surface of an elastic body 1003. The elastic body 1003 and the piezoelectric body 1004 constitute an oscillating body 120. A rotor 1001 is in contact under pressure with the other side surface of the elastic body 1003 through a slider 1002. The slider 1002 and the rotor 1001 are glued together, constituting a rotor 110.
The surface of piezoelectric body 1004 is provided with four electrodes and electrode groups, 1004a, 1004b, 1004c and 1004d as shown in FIG. 7. The driving electrode groups 1004a and 1004b are subjected to AC driving voltages which differ from each other in phase by .pi./2. The electrode 1004c is grounded. The electrode 1004d is used to take out an AC output voltage which corresponds to the oscillation of the oscillating body 120. Such conventional driving controlling device (1) controls the frequency of an AC driving voltage signal by the voltage value obtained from the monitoring electrode 1004d, or (2) controls the frequency of the AC driving voltage signal by the phase difference produced between the signal waveform of the AC voltage applied to the piezoelectric body 1004 and the voltage signal waveform issued from the monitoring electrode 1004d.
The ultrasonic motor, however, presents unstable operating characteristics during a transient period, and its monitoring electrode also produces unstable output voltage during the transient period. Transient periods here include the time when the ultrasonic motor is started, the time when the speed of the motor is changed while the motor is running, and the time when the rotating direction is reversed while the motor is running. To control the drive of the ultrasonic motor during such transient period, if the driving conditions are controlled according to the output voltage and output voltage waveform of the monitoring electrode, then the ultrasonic motor may present problems such as unstable operation or a failure to start.
The reasons set forth below are considered to be responsible for the unstable phenomena of the ultrasonic motor during such transient periods.
FIG. 2 shows the operational characteristics of the ultrasonic motor. The abscissa gives a driving frequency f, and the ordinate provides driving speed N and the phase difference .phi. produced between the waveform of the driving voltage applied to the driving electrode 1004a and the output voltage waveform of the monitoring electrode. In the figure, .phi.1 and .phi.2 depict the characteristic of the phase difference in each driving direction. With the driving voltage waveform of the driving electrode 1004b as the reference, if the driving direction is taken, for example, as the forward rotating direction when the phase of the driving voltage waveform of the driving electrode 1004a advances .pi./2, and if the characteristic of the phase difference is represented by .phi.1, then the reverse rotation results when the phase of the driving voltage waveform of the driving electrode 1004a delays .pi./2, and the characteristic of the phase difference is represented by .phi.2. F1 in the figure denotes the resonance frequency of the ultrasonic motor. The driving speed of the ultrasonic motor can be changed by changing the driving frequency f. Normally, the frequency band used for the driving control is higher than the resonance frequency F1 but its band width is 10 to 15% narrower than the resonance frequency F1. As illustrated, at frequencies in the vicinity of or lower than the resonance frequency F1, a slight change in frequency causes a significant change in the driving speed, resulting in unstable operation. If the frequency goes down further, the driving control cannot be achieved.
On the other hand, at frequencies higher than the driving frequency band, unstable operation phenomena such as a reversed driving direction is observed. In addition to the cause related to the driving frequency, a change in the driving voltage may also lead to unstable operation. Further, the operation may become unstable during a transient period when the rotational direction of the ultrasonic motor is reversed while it is running. This is considered attributable to a change in the operating characteristic of the ultrasonic motor that takes place when the rotational direction is switched, and also to the transient phenomenon caused by the switching of the phase of the driving voltage applied to the driving electrode.
Next, the output voltage of the monitoring electrode during the transient period is discussed.
FIG. 8 is a time chart which shows the output voltage of the monitoring electrode when the ultrasonic motor is started with the driving voltage and the driving frequency fixed at constant levels. When the ultrasonic motor is started with the driving voltage applied to the driving electrode at a time t1, the output voltage of the monitoring electrode gradually increases and reaches a constant voltage after a time t2. As shown in the figure, during the period from the time t1 immediately after the start to the time t2, the output voltage of the monitoring electrode is in the transient state and presents a voltage value which is different from that in the stabilized state after the time t2.
The phase difference between the driving voltage waveform of the driving electrode and the output voltage waveform of the monitoring electrode also shows a transient unstableness at the time of start. Immediately after start, in particular, a significant error is produced in the detection of the phase difference because the output voltage of the monitoring electrode is low as shown in FIG. 8.
Such transient phenomenon observed in the monitoring electrode is considered to result from the equivalent impedance of the ultrasonic motor, which changes depending on whether the motor is at rest or in the running state, causing the resonance frequency to change. There is another possible cause; if a transformer is used for the power amplifier which supplies the driving voltage to the ultrasonic motor or if a coil is connected to the driving electrode, then a transient phenomenon is considered to take place between the capacitance of the piezoelectric element of the driving electrode and the transformer or coil inductance when the driving voltage changes.
Thus, if the ultrasonic motor is controlled during such transient period according to the unstable output voltage of the monitoring electrode, then the driving frequency cannot be set within the driving frequency band wherein stable operation can be accomplished. As a result, the driving frequency turns into an unstable driving frequency as discussed above, causing the ultrasonic motor to exhibit unstable operation.